OK, you are on the right track with your thinking. An other way to think of it is that extra-territoriality just like the boarder between nations. (because it effectively is!)
Currently there is a number of companies that are large enough of warrant extra-territoriality (so many in fact that i forget them all), in game terms, any company that is AA or larger in status has extra-territoriality.
As to their size, they are as big as they need to be to encompass entity in question. So they could be as small as a single building (a few hundred square feet) , to as large as an industrial complex (a few dozen kilometers!).
An entity will have as many extra-territoriality zones as they have separate facilities. Meaning if Ares has 15 buildings spread across a town, EACH one is an Ares extra-territoriality zone!
Now, along with that extra-territoriality comes some responsibilities:
1: The area must be "clearly marked", and "clearly divided" from the adjacent territoriality.
This is a fancy way of saying they have to mark their boarders and property. This can be done as easily as posting signs, and a property divider (some sort of barrier like a fence, wall, hedge, etc). - Really, this is no different then the way any business or individual marks the boundaries of their own property now.
2: They are liable everything that happens on their property, and the effects that may have on adjacent territoriality. Which basically means, that while they can get away with just about anything on their own property, they are responsible if something harmful leaves that property. (toxins, wastes, lead)
So, how this all works in your game:
The Aztech Pyramid in Seattle is a giant mall and office building. Everyone in the city shops there. However, what many people that shop there may not realize is, the Entire Pyramid is An Aztechnology extra-territoriality Zone! Meaning that you don't deal with UCAS law and law enforcement if you commit a crime there, you deal with Aztech legal system and enforcement!
However, because the Pyramid itself is located in the heart of Seattle surrounded by the UCAS, and wants to do business with UCAS citizens it wouldn't employ lethal measures anywhere that the public would be endangered. (The security of the Pyramid would be staggered, with more and more harsh security the deeper up to the office structure you go)
Now, out in Tacoma, Aztechnology also runs cutting edge research facility and is for employees only. The building is surrounded by a concrete barrier 3 meters high, and topped with razor wire. You try to get into the facility, they will shoot you dead! And they can get away with that type of action because they have taken steps to stop collateral damage from leaving their property (the concrete wall around the facility).
The important things to remember are:
It has to be clearly marked.
It has boarder.
So a covert research facility deep in the barrens does NOT have extra-territoriality. But an office building with a giant logo on it does!
Extra-territoriality is a double edged sword for Corporations.
While it allows them to get away with virtually anything on their grounds, it also limits them to their grounds.
So if you are on a Run, and things go south and security is hot on your tail, they can only legally chase you to the edge of their property, and not an inch farther.
This also means Knight Errant and Lone Star can't follow you onto Corp Property as well, as their jurisdiction ends at the city/state property line.
(this is handy for evading capture!)
***NOTE*** THIS DOESN'T MEAN THAT THE CORPS AND THE POLICE DON'T CO-OPERATE! It just means that co-operation isn't automatic, nor explicit. It has to be sought out each time from the respective parties. But it doesn't stop a Corp from notifying the police that a BnE is in progress and assistance is requested. (So the police can take over after you're off the property)
Does that help you any?